In the field of mechanical testing of materials/(mechanical structural analysis) a method is used where force impulses are applied to the test object using a specially arranged hammer, a so-called mechanical impulse hammer. The impulse hammer is then chosen or arranged with a weight and a striking face generating force impulses whose frequency spectra are appropriate for the specific test. The hammer is further provided with a sensor, which, for each individual stroke, records the time course of the force impulse applied to the object. During the test, using one or more sensors mounted on the object, the vibration response caused by the stroke is recorded. In a subsequent analysis of recorded vibrations and applied force impulse, certain essential mechanical properties of the object can be determined. For example, the method could be used to detect the presence of cracks or other abnormalities in the test object. Additional analysis opportunities can be obtained by replacing the impulse hammer with electromagnetic or piezoacoustic actuators, allowing more arbitrary mechanical excitation forces to be applied to the test object in one or more points. Said test methods, however, require the measuring equipment to be in direct mechanical contact with the test object. In many cases, though, it is desirable to carry out the measurement without contact, at a distance from the test object. Examples of such cases are when the test object has a high temperature, is inaccessible, contaminated or subjected to high voltage. One possibility to perform such contact-free measurement is to use electromagnetic radiation interacting with the object, thereby applying the desired mechanical excitation. In these cases, the required vibration recording may also be performed without contact, for example, using a laser vibrometer.
The basic principle for producing vibrations in objects at a distance using electromagnetic (EM) waves is based on the waves' ability to transfer energy from one point to another, in combination with the ability to convert this energy, in one or more steps, into mechanical vibration energy. The conversion from EM wave into vibration can take place in a number of different ways, and there are a number of more or less well-developed methods to exploit this. For example, mention can be made of a method for generating plasma formation, so-called laser ablation. Here, a material, usually a metal, is illuminated using a pulse of very high intensity. The high intensity of the pulse causes a dielectric breakthrough in the material, which leads to plasma formation with rapid pressure rise, thereby applying a force impulse to the object's surface. Such a method is described in [Kajiwara, Itsuro; Hosoya, Naoki: Vibration testing based on impulse response excited by laser ablation; Journal of Sound and Vibration, 10 Oct. 2011, 330 (21): 5045-5057]. The disadvantage of this approach is, firstly, that the high intensity of the pulse can be directly harmful to the user, and secondly, that the irradiated material is exhausted and destroyed. Furthermore, this comprises a method based on non-linear phenomena which are themselves highly material-dependent—something that greatly complicates the analysis of the response and thus limits the scope of application. Another method known in the art is based on the electrostrictive effect. Here, molecules in the object's surface are polarized in illuminated areas. This gives rise to mechanical stress which can subsequently be read. An obvious disadvantage of this method is that its applicability depends on the polarization characteristics of the material. Most normal structural materials have low electrostrictive effect, which counteracts the general use of the method. In addition, use is sometimes made of a method called the thermoelastic (or thermoacoustic) method, which is based on a relatively brief pulse heating the surface layer of the object, whereupon the thermal expansion of the surface leads to a sudden mechanical deformation of the surface and the most proximate underlying material volume. The duration of the pulse is balanced so that heating occurs as quickly as possible but at the same time so that heating reaches a sufficient depth in order for the heated material volume to cause a sufficient tensile force to deform the underlying structure. The practical design of the method is somewhat similar to that of plasma formation, but with slightly lower, albeit significant, optical effect, which allows for measurements to be non-destructive. It has the potential for relatively high efficiency but requires, as previous methods, extensive optimization with specific design for each measured object and situation. The traceability of the produced mechanical impulse is low and the possibility of controlling the frequency spectrum of the mechanical excitation is insignificant. An example of an analysis of a mechanical component by means of the method is demonstrated in [P. Castellini; G. M. Revel; L. Scalise; Measurement of vibrational modal parameters using laser pulse excitation techniques; Measurement, Volume 25, Issue 2, March 2004, pages 163-179].